The present invention relates to real time image generation systems of the type used in simulation and interactive training and relates more particularly to a method for displaying volumetric effects such as smoke, fog and bomb bursts.
In real time image generation systems, it is typically necessary to completely build a new image during each frame interval, e.g. one fifteenth second or less, since such systems preferably permit the free changing of viewpoint in a three-dimensional world and the free changing of the position and orientation of various objects which can be seen from the viewpoint. Real time image generation systems typically operate from a database providing an inventory of objects which can be displayed. The data typically represents each object as a group of three-dimensional polygons of arbitrary sizes which can then be positioned, scaled and rotated or oriented by applying mathematical transforms. The polygons may, for example, be represented by the locations of vertices. Typically, there is associated with each surface of the polygon a texture map which permits a textured surface to be, in effect, projected on the plane of the polygon thereby to represent a surface pattern.
In building up an image, the various objects within the possible field of view are analyzed sequentially and pixel definitions are output for each screen-space pixel which the object occupies completely or partially. Since it is computationally difficult to order the objects in terms of depth or distance from the viewpoint prior to processing, there is typically provided with each object pixel definition a depth value which represents distance from the viewpoint to the object. The depth values allow control circuitry associated with a depth buffer to determine which objects are nearer the viewpoint should thus control the color and brightness of a given pixel and allows for mixing of colors when objects only partially occupy a pixel and for performing hidden surface elimination. Prior art systems, however, do not provide for the displaying of volumetric effects such as smoke, fog and bomb bursts where the contribution from the effect should depend upon the amount of the affected volume which is in front of any object being viewed and which should also be modified if the viewpoint is within the volumetric effect.
Among the objects of the present invention may be noted the provision of a real time image generation system which enables the displaying of volumetric effects such as smoke, fog and bomb bursts; the provision of such a system which allows the effect to be modified as a function of the distance through the affected volume between the viewpoint and an object being viewed; the provision of such a system which allows the effect to be realistically modified for viewpoints within the affected volume; the provision of such a system which does not require greatly expanded buffer memory; the provision of such a method which is highly reliable and which is of relatively simple and inexpensive implementation. Other objects and features will be in part apparent and in part pointed out hereinafter.